Meters and other Electrical Instruments. 7 



B was measured at about the end of one of the fixed pieces 

 where the movable piece faces it. 



(8) Ayrton and Perry Magnifying Spring Voltmeter. — The 

 small iron tube, which is surrounded by a coil and pulled 

 down by it, is 7*2 centim. long, and the iron has a section of 

 about 0*12 sq. centim. The number given in the table is 

 the average B in the iron for the whole length of the tube 

 for a load of 15 volts, i. e. 08 of fall load. 



Fiff. 2. 



(9) Richard Recording Ammeter. — This has two solid iron 

 cores (each 4 sq. centims. cross- section) round which the 

 current is carried by a single turn of copper strip ; an iron 

 armature carrying the pointer is attracted by these cores. B 

 was measured between one core and the armature. It will 

 be seen that the total flux is large ; thus a strong deflecting 

 force is obtained which makes the friction of the recording 

 pen of less account, but on the other hand much error from 

 hvsteresis comes in. 



